It has long been known that halogen containing organic polymers, such as polyvinyl chloride are subject to deterioration or degradation when exposed to heat and light.
Halogen containing organic polymers are also subject to deterioration or degradation during processing. The difference in processing temperature and degradation temperature for halogen containing organic polymers such as polyvinyl chloride and chlorinated polyvinyl chloride is very small. Thus any mistake either mechanically or chemically will result in degradation.
It is believed that when halogen containing polymers degrade, a halide acid such as HCl is removed from the polymer. The emitted acid attacks the components of the processing machine and more importantly catalyzes further elimination reactions or degradation of the polymer. The resultant dehydrohalogenated product which now contains an additional allylic chlorine is prone to further degradation and is capable of violent autocatalytic decomposition or unzipping.
Various chemical compounds and compositions have been developed to stabilize the halogen containing organic polymers against such deterioration. Ideally a stabilizer should replace labile halogens by displacement with a ligand less easily removed by heat or other degenerative means, work as an antioxidant and convert the evicted halogen to an innocuous state.
Halogen containing organic polymer stabilizers have generally been directed toward stabilization against heat degradation, such as that encountered during processing of the polymer and its fabrication into articles. Some of the heat stabilizers which are commercially available are described in various trade publications and reference materials such as the 1993 Modern Plastics Encyclopedia and Plastics Additives Handbook, edited by R. Gachter and H. Muller, Hanser Publishers.COPYRGT. 1983, pp. 204-230 which are incorporated herein by reference.
The more common heat stabilizers are organotin compounds such as mono and dialkyltin carboxylates, mono and dialkyltin mercaptides, mixed metal stabilizers such as barium, cadmium, barium/zinc or cadmium/zinc salts of carboxylic acids and phenols, lead stabilizers such as tribasic lead sulfate, dibasic lead phosphite, dibasic lead phthalate, dibasic lead stearate, dibasic lead carbonate and lead stearate and metal free stabilizers such as 2-phenylindole and aminocrotonates.
Other compounds can be used to stabilize halogen containing organic polymers but these compounds have gained only modest importance in the art such as antimony-trismercaptides.
It is also known in the art to use costabilizers to enhance the stability of halogen containing organic polymers. Some common costabilizers are organic phosphites such as diphenyl-decylphosphite and phenyl-didecylphosphite, epoxy compounds such as epoxidized soybean oil, epoxidized castor oil and epoxidized linseed oil, polyols such as pentaerythritol, dipentaerythritol, trismethylolpropane and sorbital, and antioxidants such as bisphenol A, 2,6-di-tertbutyl-4-methylphenol and octadecyl 3(3,5-di-tertbutyl-4-hydroxyphenyl)propionate. Other costabilizers or stabilizer enhancers are described in U.S. Pat. No. 4,593,059 which discloses a diester of an ethylenically unsaturated dicarboxylic acid having one carboxyl group on each carbon atom of the ethylene group; U.S. Pat. No. 5,073,584 which discloses a combination of an alkali metal alumo silicate and an isocyanuric acid derivative; U.S. Pat. No. 4,060,508 discloses a combination of an inorganic stabilizer represented by the formula MO.sub.n SiO.sub.2 wherein M is a group II or IV metal and n is from 0.3 to 5 and an organic additive; and U.S. Pat. No. 5,234,981 which discloses a combination of an organic zinc or cadmium compound, a calcium, magnesium, barium or lanthanide compound and an amorphous basic aluminum magnesium carbonate. The aforementioned patents are incorporated herein by reference.
Other costabilizers known in the art are molecular sieves or zeolite compounds. For example U.S. Pat. No. 4,000,100, incorporated herein by reference, discloses the use of Zeolite A molecular sieves which have approximately 18-25% water content with conventional inorganic, organometallic or organic stabilizers. Unfortunately, the high water content of these zeolite compounds often evaporates during processing resulting in unwanted orange peel products or products with a grainy finish. A further disadvantage of using zeolites as a stabilizer for halogen polymers is that many of the zeolites incorporate sodium which can impart an undesirable reddish color to the polymer product.
With the increased use of halogen containing organic polymers it has become increasingly important to develop a stabilizer which will stabilize the halogen containing polymer during processing and also to provide a smooth and uniform finish for the final product.